Interleaved,high series capacitance coils



Nov. 3, 1970 R. l. vAN NICE 3,538,471

I NTERLEAVED, HIGH SERIES CAPACITANCE COILS l 4 Filed April so, 1969 A l 2 sheets4sheet 1 QX L' l2 E L |8\ Y l I lg i A5|B|o A4|B9|A3 Bela? A2|Be A| B5 Ao 'Z g 64 sofi- I 52` ,se I FIG. I. I B5 Aol se Al B7 A2 B3 A3 B9 A4fa|o A5 3Q I` 22\ 2e` l 66/ I BIoAs B9 A4 Be A3 A2 B7 AI B6' Ao B5 2g' FIG 2 ;;:Bu A5 Blo A4 B9 A5 Be B7 A2 Be Al B5 Ao ZQ FIG-3 :12| I l I 22\ A5 BIO A4 B9 A388 B7 A2 B6 AI B5 AO AO B6 AI B7 A288 A3 B9 A4 BIO A5 BII A6 2 .rh-Nov.. 3,1970 R. l. VAN NICE I 35.538,471

u INTERLEAVED, HIGH SERIES CAPACITANCE COILS yFiled April so, 1969i 2 sheets-sheet z WITNESSES' INVENTOR s r Rser'r I. Von Nice JZ( fa/f Q Q67/ C/ mw w TORNEY United States Patent O U.S. Cl. 336-70 6 Claims ABSTRACT OF THE DISCLOSURE An electrical transformer of the core-form type which includes a plurality of electrically connected pancake coils of the singly interleaved, high series capacitance type. Each of the pancake coils have a plurality of conductor turns formed of liirst and second radially interleaved coil sections which are interconnected to provide a single series path through each pancake coil, which traverses the pancake coil build twice in the same radial direction. In at least one of the pancake coils, the relative radial sequence of the irst and second coil Sections is reversed at a predetermined point in the coil build, which when applied to a pancake coil having an odd number of turns makes the coil react to a surge potential similar to a pancake coil having an even number of turns, and when applied to a pancake coil having an even number of turns makes the coil react to a surge potential similar to one having an odd number of turns.

BACKGROUNDv OF THE INVENTION Field of the invention The invention relates in general to electrical power transformers of the core-form type, and more specifically to electrical transformers having pancake or disc-type coils of the interleaved turn, high series capacitance type.

Description of the prior art The electrical windings of certain types of electrical inductive apparatus are formed of a plurality of pancake type coils disposed in spaced side-by-side relation, and electrically connected to form a winding. In order to more uniformly distribute surge potentials across such a winding, each pancake coil may have a plurality of sections, the turns of which are radially interleaved with one another, and the sections are interconnected to place turns from an electrically distant portion of the coil or winding between electrically connected turns. This method of constructing a pancake coil or winding to increase its through or series capacitance is termed interleaving.

A most successful interleaving arrangement from the standpoint of simplicity, cost, and electrical results, is the single interleaved pancake coil disclosed in U.S. Pat. 3,090,022, issued May 14, 1963, which is assigned to the same assignee as the present application. The singly interleaved pancake coil has first and second coil sections, the turns of which are radially interleaved, with the two sections being interconnected to provide a series path through the coil which traverses the coil build twice in the same radial direction. In other words, rst and second conductors, each having rst and second ends, are wound together, starting with their first ends adjacent one another, to provide two coil sections, and then the first end of one coil section is connected to the second end of the other coil section.

It has been found that pancake coils do not always react in a similar manner to a surge potential, with pancake coils having an odd number of interleaved turns Mice sometimes behaving quite differently than pancake coils having an even number of turns. Further, a given pancake coil with an even or odd number of turns may have a different reaction to a surge potential, depending upon the specific transformer construction, insulating clearances and other constructional details. It has been found that in certain transformer constructions that coils having an odd number of turns provide better electrical results when subjected to a surge potential than those having an even number of turns, such as having a lower voltage to ground, a lower maximum turn-to-turn voltage, a lower maximum coil-to-coil voltage, and a lower voltage between the turns of the coil and its interleaving connection. On the other hand, in other transformer constructions, pancake coils having an even number of turns provide improved electrical results when subjected to a surge potential, compared with coils having an odd number of turns. In order to achieve the best electrical results during a surge potential, some parts of a winding may require coils having an odd number of turns, while other parts of the winding may require pancake coils having an even number of turns. For example, tapped sections may require, for best surge potential results, an even number of turns, while coils at other locations in the winding may require an odd number of turns.

These requirements for an odd or an even number of turns to provide the best surge protection characteristics, however, do not take into consideration the design of the transformer to provide the desired steady state electrical characteristics, and it would complicate the transformer design to change from an odd to an even number of turns, or vice versa, at selectedlocations in the winding, just to optimize its surge characteristic. Thus, it would be desirable to be able to take advantage of the improved surge distribution characteristics of coils having an even or an odd number of turns, as required at certain locations in the winding, if this could be accomplished without changing the actual number of turns as required by the steady state transformer design. Or, in certain instances, it would be desirable to be able to actually change the physical number of turns from an odd number to an even number, or vice versa, if this could be accomplished without changing the electrical elect of these coils during a surge potential.

SUMMARY OF THE INVENTION Briefly, the present invention is a new and improved winding or coil construction, for singly interleaved pancake coils, which when applied to a singly interleaved pancake coil having an odd number of turns causes it to exhibit surge characteristics similar to a pancake coil having an even number of turns, and when applied to a singly interleaved coil having an even number of turns causes it to exhibit surge characteristics similar to a pancake coil having an odd number of turns.

This new and improved construction for singly interleaved pancake coils requires the reversal of the relative radial sequence of the two radially interleaved coil sections which make up a singly interleaved pancake coil, with the reversal occurring at substantially the mid-point of the build dimension of the pancake coil. Thus, instead of the two radially interleaved coil sections having the same radial sequence across the coil build, such as the sequence A-B, A-B, A-B, where the letters A and B represent the two radially interleaved coil sections, at some point in the coil build, such as at the mid-point, the sequence is changed from A-B to B-A. Therefore, the transformer may be designed with singly interleaved pancake coils having an even or an odd number of turns, as required by the steady state design requirements, without regard to their effect during a surge potential.

Coils having a predetermined odd number of turns may be changed to have the surge effect of coils having an even number of turns, and vice versa, without actually changing the physical number of turns in the pancake coil. Thus, all of the coils may have the same number of turns, and thus the same build dimension, while still changing the surge characteristics of certain predetermined coils. Or, it will be possible to change the number of turns in the coils from an odd to an even number, or vice versa, as required to maintain predetermined build dimensions, such as when grading the insulation thickness between turns of the pancake coils across the winding, while maintaining a predetermined reaction characteristic of the coils to a surge potential.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIG. 1 is a partial sectional elevation of the magnetic core and windings of a transformer having a pancake coil with an even number of turns, constructed according to the teachings of the invention;

FIG. 1A is a diagrammatic or schematic representation of a plan view of one of the pancake coils shown in FIG. 1;

FIG. 2 is a fragmentary, diagrammatic view of a pancake coil having an even number of turns constructed according to another embodiment of the invention;

FIG. 3 is a fragmentary view of a pancake coil having an odd number of turns constructed according to the teachings of the invention;

FIG. 4 is a fragmentary view of a pancake coil having an odd number of tums constructed according to another em'bodiment of the invention;

FIG. 5 is a fragmentary view of a pancake coil having an even number of turns constructed according to the teachings of the invention, connected to a singly interleaved pancake coil having an odd number of turns; and

FIG. 6 is a fragmentary yview of a pancake coil having an even number of turns connected to a pancake coil constructed according to the teachings of the invention, which coil has an odd number of turns.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, and FIG. 1 in particular, there is shown a partial sectional elevation of a magnetic coreewinding assembly 12 of a transformer 10 constructed according to the teachings of the invention. The magnetic core-winding assembly 12 includes high and low voltage windings 14 and 16, respectively, concentrically disposed about a leg 18 of a magnetic core assembly 20, in what is commonly referred to as coreform construction. Transformer may be either single or polyphase, with only one phase being illustrated in order to simplify the drawings. Transformer 10 may be of the isolated winding type, with each end of the high voltage winding 14 being adapted for connection to a source of electrical potential, or one end may be adapted for connection to ground, depending upon the particular requirements of the application. Transformer 10 may also be of the auto-transformer type, if desired. The high and low voltage windings 14 and 16, respectively, are concentrically or coaxially disposed relative to an axis or center line 22, with the windings only being shown on one side of center line 22, as their views on the other side would be similar.

Low voltage winding 16 may -be of any conventional type, having a plurality of conductor turns 24 which are insulated from the magnetic core `20 and high voltage winding 14 by insulating means 26.

High voltage Iwinding 14 includes a plurality of pancake or disc type coil sections of which pancake coils 28 and 30 are shown adjacent line terminal L1, and connected to line vconductor L. It is to be understood that the invention applies to any plurality of pancake coils, with additional pancake coils being indicated generally at 52.

Each of the pancake coils, such as pancake coil 28, includes a plurality of conductor turns wound to provide an opening for receiving winding leg 18 of magnetic core 20 and low voltage winding 16, forming a substantially disc shape having rst and second outer major opposed surfaces and a predetermined radial build or outside diameter. The various pancake coils are stacked with their openings in alignment, with their major surfaces in spaced parallel relation with one another to form cooling ducts between the adjacent coils, such as cooling duct 56 between pancake coils 28 and 30. The plurality of pancake coils are electrically connected in series, with either start-start, finish-finish connections as shown in FIG. 1, or with finish-start connections. As used in this specification, the start of a pancake coil is always the end of one of its inner turns, and the finish of the winding is always the end of one of its outer turns, regardless of where the electrical circuit first enters the pancake coil.

The magnetic core-Winding assembly 12 may be disposed in a suitable casing or tank (not shown), which may be filled to a predetermined level with a uid insulating and cooling medium, such as oil or SP6.

A surge potential applied to line terminal L of winding 14 will tend to concentrate on the first few pancake coils at this end of the winding, increasing the voltage between adjacent pancake coils at this end of the winding, increasing the voltage from these pancake coils to ground, and increasing the turn-to-turn voltage in these pancake coils. The extent of this non-linearity in the distribution of the surge potential across the winding and the turns thereof, is determined by the distribution constant oc of the winding, with the smaller the distribution constant, the more linear the distribution of surge potentials across the winding. The distribution constant oc may be made smaller by increasing the series capacitance of the pancake coils and the winding.

The usual approach in reducing the distribution constant o: is to increase the series capacitance of the pancake coils and the winding by forming the pancake coils in two or more radially interleaved sections and connecting the sections to interleave the turns thereof, which changes the turn-to-turn capacitances from a series network to a series-parallel network, which substantially increases the series capacitance of the pancake coils and the winding.

A very successful interleaving arrangement or construction is disclosed in the hereinbefore mentioned U.S. Pat. 3,090,022, which singly interleaves the pancake coil by radially winding two conductors together having inner or first ends and outer or second ends, which forms two 4radially interleaved sections in the coil, then, the inner end of one of the coil sections is connected to the outer end of the outer coil section, with the connecting conductor being called the interleaving connection. Singly interleaved pancake coils may be constructed such that the total number of turns in the two interleaved sections is an odd number, or an even number. Surprisingly, it has been found that whether a singly interleaved pancake coil has an even or an odd number of turns may make a substantial difference in how the pancake coil reacts to a surge potential. Further, it has been found that in certain situations a pancake coil having an odd number of turns will provide lower maximum stresses when subjected to a surge potential than one having an even number of turns, and in other situations the reverse will be true. It has also been found that in a given winding that pancake coils having an odd number of turns and pancake coils having an even number of turns may both be required in order to provide optimum surge performance. For example, pancake coils in the tapped sections may perform better when subjected to a surge if they have an even number of turns, while the pancake coils in the remaining portions of the winding may react more favorably to surge potentials if they have an odd number of turns. Or, the pancake coils at the line end or ends of the winding may perform better with an odd number of turns. and those more remote from the line end, or ends, may perform better when subjected to a surge potential if the coils have an even number of turns. With a given winding design, it may be determined experimentally where to use pancake coils having an even number of turns and where to use pancake coils having an odd number of turns.

Changing the number of turns in pancake coils to optimize their surge performance, however, places severe restrictions on the steady state design of the transformer, and may also create constructional problems due to undesirable diiferences in the build dimensions of the coils. This invention makes it possible to design the pancake coils to have the number of turns required for steady state and constructional purposes, and then improve the surge performance of those even turn coils which should have an odd number of turns, and those odd turn coils which should have an even number of turns, by a constructional change which does not affect the number of turns in the coil, nor does it affect the build dimension of the coil from its inner opening to its outer maximum diameter. This constructional change when applied to a pancake coil having an even number of turns, causes it to react to a surge potential as though it has an odd number of turns, and when applied to a pancake coil having an odd number of turns causes it to react to a surge potential as though it had an even number of turns.

In general, the construction change involves reversing the relative radial sequence of the two radially interleaved coil sections of a pancake coil at about the midpoint of the coil build dimension. Thus, as shown in FIG. 1, both pancake coils 28 and 30 have ten turns, an even number. Pancake coil 28, in this example, has the relative radial sequence of the two radially interleaved sections reversed at a point mid-way between its inner and outer diameters, to change the reaction of pancake coil 20* to a surge potential to that of a pancake coil having an odd number of turns. This has been accomplished while maintaining the build dimension of the pancake coils 28l and 30 the same, even though pancake coil 30 is constructed according to the teachings of the prior art, without changing its reaction to a surge potential.

More specifically, pancake coil 28 has first and second radially interleaved sections, which will be called the A and B sections, and since in this example the pancake coils 28 and 30 are interconnected with start-start connections, as used in a start-start, linish-iinish connecting winding, the A and B circuits start at the ends of the outer two turns of the coil 28 and spiral inwardly together. If the line terminal L1 is connected to the end of the outermost turn, which will be called the A section, the A section spirals inwardly appearing at every other turn across approximately one-half of the coil build. The coil turns are given the letter of its associated section, and a number to denote the number of turns from the line end of the pancake coil. At approximately the mid-point of the coil build, the A section is connected via conductor 60 into what would normally be the B section of the coil, which places two turns from the other section between its turns numbered A2 and A3, instead of having the conventional one turn between these conductors. The A circuit then continues to spiral inwardly in its new position, appearing at every other turn until reaching the end of thel inner turn of its section, which is the innermost turn numbered A5. The circuit then returns to the start of the next to the outermost turn via interleaving conductor 62, and again spirals inwardly appearing at alternate turns, except at the mid-point of the coil build where it has two turns disposed side-by-side, and then it spirals inwardly through the remaining portion of the coil build, appearing at every other turn until reaching the end of the next to the innermost turn at B10.

While the cross sectional view of pancake coil 28 makes it appear that conductor 60 crosses two conductors, in actual practice it only crosses one. This is more clearly illustrated in FIG. 1A which is a schematic or diagrammatic view of the one of the major sides of pancake coil 28, illustrating that the reversal of the relative radial sequence of the two radially interleaved sections requires that only one conductor be severed, with the other conductor proceeding to its new position without being broken. The severed conductor then crosses over the other conductor via connection 60, to enter its new position. While this reversal of relative radial positions of the two interleaved sections appears to be what is known in the art as a transposition, it is not a transposition. A transposition is made in circuits having a plurality of parallel connected conductors to reduce circulating currents in the parallel circuits. In the instant invention, the pancake coils have only a single series path therethrough.

When pancake coil 28 has been completed, the circuit proceeds to the next adjacent pancake coil 30` with startstart connection 64, which may enter the end of the next to the innermost turn of pancake coil 30, as illustrated, or the end of the innermost turn, as desired. As illustrated in FIG. l, pancake coil 30` is of the conventional singly interleaved construction, having iirst and second radially interleaved sections, referenced the A and B sections, with interleaving conductor 66 connecting the end of the outermost turn A5 of the A section with the end of the innermost turn B5 Of the B section.

FIG. 2 is a diagrammatic representation of a pancake coil 28' having an even number of turns, which is modification of the even numbered turn pancake coil 28 shown in FIG. 1. In this embodiment, the conductor L lirst enters the end of the next to the outermost turn, instead of the end of the outermost turn, as illustrated in FIG. 1. The conductor 60 makes the required reversal of the relative radial sequences of the two radially interleaved sections, and conductor 62 connects the end of the next to the innermost turn A5 with the end of the outermost turn B5, forming the interleaving connection of the pancake coil.

FIG. 3 is a diagrammatic view of a pancake coil 70 constructed according to the teachings of the invention, which has an odd number of turns, eleven in this instance,

instead of an even number of turns as illustrated in FIGS.

l, 1A and 2. Pancake coil 70` is otherwise structurally similar to pancake coil 28, with conductor 72 entering the end of the outermost turn and spiralling inwardly, appearing at every other turn until substantially the mid-point of the coil build, at which point two turns from the other section are disposed between its turns, due to the reversal of the relative radial positions of the interleaved coil sections, signified by conductor 74 interconnecting turns A2 and A3. At the end of the next to the innermost turn, the circuit proceeds via interleaving connection 76 to the end of the next to the outermost turn, and it then spirals inwardly, appearing at every other turn, until reaching the point where the radial sequence of the interleaved section is reversed, at which point two of its turns appear side-by-side. The B section then continues to spiral inwardly appearing at every other turn until reaching the end of the innermost turn at turn B11.

FIG. 4 is a diagrammatic representation of pancake coil 70 having an odd number of turns, which is a rnodiiication of the pancake coil 70 shown in FIG. 3. Instead of the conductor irst entering the end of the outermost turn, as in FIG. 3, conductor 72 enters the section which appears at the end of the next to the outermost turn, and then spirals inwardly appearing at every other turn until the mid-point of the coil build, at which point it has two adjacent turns, and then it continues to appear at every other turn until reaching' the end of the innermost turn A6. The appearance of two adjacent turns of the A section was due to the reversal of the relative radial sequence of the two interleaved coil sections, signified by a conductor 74. The circuit then continues via interleaving connection 76', connecting the end of the innermost turn A6 with the end of the outermost turn B6, with the circuit again spiralling inwardly, appearing at every other turn until reaching the point at which the relative radial sequence of the two coil sections is reversed, at which point the B section has two turns from the A section disposed between two of its electrically connected turns B8 and B9. The B circuit continues to spiral inwardly, appearing at every other turn until reaching the end of its innermost turn B11.

In FIG. 1, the two pancake coils 20 and 30 each have an even number of turns, with pancake coil 28 being constructed according to the teachings of the invention `to cause it to react to a surge potential similar to a pancake coil having an odd number of turns. FIG. illustrates two pancake coils, one of which has an even number of turns and the other of which has an odd number of turns, with the pancake coil having an even number of turns being constructed according to the teachings of the invention to react to a surge potential in the manner of a pancake coil having an odd number of turns.

More specifically, FIG. 5 illustrates pancake coils 80 and 82, which are interconnected with start-start connections, for use in a winding which has its pancake coils connected with start-start, iinish-finish connections, with pancake coil 80 having an even number of turns, and pancake coil 82 having an odd number of turns. Pancake coil l80 is constructed accordirig to the teachings of the invention, to change its reaction to a surge potential from that 0f a coil having an even number of turns to a coil having an odd number of turns. Pancake coil 82 in this instance, is of conventional single interleaved construction. Thus, pancake coils 80 and 82 'will both have a similar reaction to a surge potential, even though pancake coil 82 has one more turn than pancake coil 80. This construction is useful when it is not desirable to change the surge characteristics of the pancake coils from one pancake coil to the next, but where it is desirable to change the number of turns or build dimension. Or, when the insulation between turns of one pancake coil is different than the insulation between the turns of another pancake coil the number of turns may be changed to provide the same radial build dimension, and by using the teachings of this invention, the surge characteristics of the coil may be maintained substantially the same even though odd and even numbers of turns are utilized in interconnected pancake coils.

As illustrated in FIG. 5, pancake coil 80 has two interleaved coil sections, labeled the A and B sections, with the circuit first entering the end of the outermost turn A0 and spiralling inwardly, appearing at every other turn until reaching the mid-point of the coil build, where two turns from the B section are interposed between its turns A2 and A3. Conductor 84 cbnnects the two radial portions of the A sections, and reverses the relative radial positions of the turns of the A and B coil sections. At the end of the innermost turn A5, the circuit is brought back to the end of the next to the outermost turn B5, via interleaving conductor 86, and the circuit spirals inwardly via section B, appearing at every other turn, except where the reversal of the relative radial positions of the coil sections occurs, at which point turns B7 and B8 appear side-by-side. The B circuit then continues to spiral inwardly, appearing at every other turn, until reaching the end of the next to the innermost turn B10, where it proceeds to the next pancake coil 82 via start-start connection 88. Connection 88 may enter the end of the innermost turn, as shown, or the end of the next to the innermost turn, as desired, with pancake coil 82 being constructed according to an embodiment of conventional single interleaving practice. In other words, pancake coil `82 has two radially interleaved coil sections, with the end of the outermost turn A6 being connected to the end of the next to the innermost turn B6 via interleaving connection 90.

While all of the pancake coils have been illustrated in the figures as being start-start, finish-finish connected, they may be connected finish-start, if desired. FIG. 6 is a diagrammatic view of pancake coils and 102 connected finish-start, with pancake coil '100 having an even number of turns and pancake coil 102 having an odd number of turns, with pancake coil 102 being constructed to react to a surge potential in the manner of a pancake coil having an even number of turns.

More specifically, pancake coil 100 has first and second radial sections, called the A and B sections, which in this example spiral outwardly, with conductor 104 entering the end of the innermost turn A0 and spiralling outwardly, appearing at every other turn until reaching the end of the next to the outermost turn A5, where it returns via interleaving conductor 106 to enter the end of the next to the innermost turn B5. The circuit again spirals outwardly, appearing at every other turn until reaching the end of the outermost conductor B10. Thus, pancake coil 100 is constnucted according to conventional single interleaving practice. The circuit then proceeds to pancake coil 102 via finish-start connection 108, which may enter the end of the next to the innermost turn thereof, as shown in FIG. 6, or the end of the innermost turn, as desired, and the Icircuit spirals outwardly appearing at every other turn until reaching the point where the relative radial positions of the two coil sections are reversed. At this point, turns A2 and A3 of the A section appear side-by-side, with the reversal of the sections being signified by conductor 110, and then the A circuit again continues to spiral outwardly, appearing at every other turn until reaching the end of the outermost turn A6. The circuit then proceeds via interleaving connection 112 to the end of the innermost turn B6, with the circuit spiralling outwardly, appearing at every other tum until reaching the side-by-side turns A2 and A3 at which point turns B8 and B9 are disposed on opposite sides of the turns A2 and A3, and then the B circuit interleaves the A circuit for the remaining portion of the radial build of the pancake coil.

In summary, there has been disclosed a new and improved construction for singly interleaved, high series capacitance pancake coils, which enables a pancake coil having an even number of turns to react to a surge potential in the manner of a pancake coil having an odd number of turns, and a pancake coil having an odd number of turns to react to a surge potential in the manner of a pancake coil having an even number of turns. Therefore, a transformer may be designed with the desired constructional details and dimensions, and desired number of turns per pancake coil, without regard to the reaction of the coils to a surge potential, as determined by whether the coils have an even or an odd number of turns. This is true since any desired pancake coil in the winding may be constructed according to the teachings of the invention, to possess the surge characteristics of an odd or an even numbered turn pancake coil, as desired.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. An interleaved turn, high series capacitance pancake coil, comprising:

a plurality of radially disposed conductor turns, providing a predetermined build dimension,

said conductor turns including -rst and second conductors each having first and second ends,

means connecting the second end of one of the conductors to the first end of the other of the conductors, providing a single series path which traverses the build dimension of the pancake coil twice in the same radial direction,

said iirst and second conductors appearing at adjacent turns in a predetermined radial sequence across a predetermined portion of the build dimension,

and means reversing the relative radial positions of the lirst and second conductors across the remaining portion of the build dimension.

2. The pancake coil of claim 1 wherein the pancake coil has an even number of turns.

3. The pancake coil of claim 1 wherein the pancake coil has an odd number of turns.

4. An electrical winding comprising:

a plurality of pancake coils of the interleaved turn,

high series capacitance type, disposed in spaced, sideby-side relation,

each of said pancake coils having a plurality of radially disposed conductor turns providing a predetermined build dimension, said conductor turns including rst and second radially interleaved conductors each having lirst and second ends, means connecting the second end of one of the conductors to the first end of one of the other conductors, providing a single series path through each pancake coil which traverses the build dimension of the pancake coil twice in the same radial direction,

at least certain of said pancake coils including means which reverses the relative radial sequence of the interleaved first and second conductors at a predetermined point in the radial build of said certain coils,

and means interconnecting adjacent pancake coils to provide a series path through the plurality of pancake coils.

5. The electrical Winding of claim 4 wherein the pancake coils are interconnected with start-start, finish-finish connections.

6. The electrical winding of claim 4 wherein the pancake coils are finish-start connected.

References Cited UNITED STATES PATENTS 512,340 l/l894 Tesla 336-69 XR 3,090,022 5/ 1963 Stein 336-70 THOMAS, J. KOZMA, Primary Examiner 

